There are 2 different blood screens you can get done- 1st trimester screening is the one done in conjunction with the NT scan.

2nd trimester maternal serum screening is a blood test taken between week 14 to 20 (optimal 15-17 weeks) and gives you a risk value for trisomy 21, trisomy 18 and neural tube defects such as spina bifida. If you get bloods taken before week 15, you may not get a caculated risk for neural tube defects.